Liquid crystal elements have been put into practical use in various applications, such as watches, table calculators, personal computers, and miniaturized liquid crystal TV on account of their electro-optic effects. Most of the liquid crystal display elements now in practical use utilize dielectric alignment effect of nematic liquid crystals or cholesteric liquid crystals. However, these liquid crystals have insufficient multiplex driving characteristics to cope with an increase in display capacity and are therefore limited in possibility of broadening of a display area. Moreover, they have a very slow electro-optic response and require a switching time from several tens milliseconds to several hundreds milliseconds. These problems have been a technical bar to development of liquid crystal displays with large capacity, large area, and high rate of response. It has thus been keenly demanded to develop a novel liquid crystal material by which these technical problems can be overcome.
Ever since it was reported that ferroelectric liquid crystals prepared from optically active compounds can be driven at a high speed on the order of microsecond and maintain their stable molecular states to exert a memory effect, a number of ferroelectric liquid crystals including Schiff base compounds, biphenyl compounds, benzoic ester compounds, and pyrimidine compounds have been synthesized, studied, and partly put into practical use to date. However, Schiff base compounds have difficulties on practical use due to the unsaturated double bond thereof and lability to water. Use of pyrimidine compounds as liquid crystal has been proposed in JP-A-57-95965 and JP-A-61-129169 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), but the proposed pyrimidine compounds do not have a sufficiently broad range of phase transition temperature inclusive of room temperature. Up to date, there has been developed no liquid crystal material which, even when used alone, exhibits a liquid crystal phase in a broad temperature range from low to high, has a low viscosity coefficient and is expected to have a high response rate, has a proper birefringence for obtaining a high display contrast, and exhibits a series of phases for realizing satisfactory orientation characteristics. That is, currently available liquid crystal materials are multi-component mixed liquid crystal compositions comprising several different kinds of liquid crystal compounds. Thus, from the aspect of liquid crystal materials, development of a novel liquid crystal having high performance properties has been demanded.
On the other hand, various electro-optic devices using ferroelectric liquid crystals have hitherto been developed. For example, several high speed electro-optic devices using ferroelectric liquid crystals have been proposed to date. Typically, such devices include an element in which a twisted structure is untwisted by the force of wall surfaces, and two direction of molecular orientation in parallel in the wall surfaces are varied by changing the polarity of an applied electric field as described, e.g., in JP-A-56-107216.
The use of a compound showing ideal two states (bistable compound) having an electric field response waveform as shown in FIG. 1 is prerequisite in the above-described devices. However, such a compound exhibiting ideal two states is not yet available. The so far synthesized bistable liquid crystals have a response waveform as shown in FIG. 2, not as shown in FIG. 1. When the state-of-the-art liquid crystals having a response waveform as shown in FIG. 2 are used, for example, in light switching circuits, since transmission gradually changes as the applied voltage changes from negative to positive, the desired results cannot be sufficiently achieved simply by changing the applied voltage between "on" and "off". Moreover, currently available bistable liquid crystals have difficulty in reaching a mono-domain state in their Sc* phase without an applied voltage, i.e., in reaching an ideal molecular orientation state, and easily undergo defect or a molecular orientation disturbance called twist. Thus, it has been difficult to achieve the above-stated ideal two states of molecular orientation over a wide area.
Further, because the threshold value (voltage at which luminance changes by a prescribed value) is low, dynamic driving is liable to suffer from a reduction in contrast or a reduction in the viewing angle.
Furthermore, these conventional bistable liquid crystals do not exhibit a hysteresis loop as shown in FIG. 1, but exhibit hysteresis as shown in FIG. 2 so that they have no memory effect. Therefore, it is necessary to continue applying a voltage of .nu..sub.3 as shown in FIG. 2 or continue applying a high frequency for the liquid crystal to maintain a stable response in the S*c phase, which, in either case, results in a considerable energy consumption.
Thus, conventional electro-optic devices have many defects which need to be overcome, notwithstanding the strong demand for devices which make effective use of the characteristics of electro-optic devices to use an applied electric field to achieve molecular orientation of ferroelectric liquid crystals.